By increasing their combat and magical skills inSkyrim, players can open new dialogue options with NPCs and receive unique rewards not available to beginners, and Sigil Stone is one of them. The usefulness of this post-story item is extremely difficult to overestimate because, in addition to its value as an ingredient forcrafting Daedric Armor, players can use it to obtain unique spells such as Flame Thrall. However, getting this item is not easy.
Players who neglect to improve Conjuration as they travel will have a hard time since the minimum level of this stat to obtain a Sigil Stone is 90. However, this is not the only requirement, so we are here to guide you along this path.

Where to Get a Sigil Stone in Skyrim
The only way to get Sigil Stone inSkyrimis as part of the Conjuration Ritual Spell late-game quest received from Phinis Gestor inthe College of Winterhold. The main requirement to start this quest is a Conjuration level of 90 or higher since Phinis Gestor will offer you a new dialogue option only after this.
Having almost reached the pinnacle of learning Conjuration, you decide to find out if you are missing something important or more powerful. Convinced of your strength, Phinis Gestor will invite you to challenge Unbound Dremora and take the Sigil Stone from it. Here are the steps you need to go through:

It is worth noting that after completing the Conjuration Ritual Spell quest, Phinis will also open up the opportunity for players to purchasemaster-level Conjuration spells, which can be extremely useful in the later stages of gameplay.